Superconductivity was discovered by Dutch physicist Heike Kamerlingh Onnes on Apr. 8, 1911 in Leiden, Netherlands. It is a phenomenon of exactly zero electrical resistance and expulsion of magnetic fields that occurs in certain materials when cooled below a critical transition temperature that is a unique characteristic of each superconductive material. Until 1986, physicists believed that superconductivity was forbidden at temperatures above about 30 K, and utilization of known superconductive materials was extremely expensive and highly impractical. However, in that year, superconductivity was discovered in a lanthanum-based cuprate perovskite having a critical transition temperature of 35 K. This discovery quickly led to the finding that by replacing the lanthanum with yttrium (i.e., YBCO) the critical transition temperature could be raised to 92 K. Other such materials having critical transition temperatures of greater than about 30 K have since been discovered or developed and are generally termed high temperature superconductors.
The development of high temperature superconductors having a critical transition temperature of greater than 77 K has been significant as it has allowed liquid nitrogen to be utilized as a refrigerant. As liquid nitrogen can be produced relatively cheaply, even on-site, previous problems with the use of liquid helium have been overcome and the practical application of superconductive materials has been greatly expanded.
Unfortunately, the ceramic high temperature superconductors are complex, expensive to produce and often toxic. As such, the development of alternative superconductive materials that are less expensive and non-toxic has been of great interest.
One of the most promising materials for development as useful superconducting materials are carbon based materials such as fullerenes. Fullerenes, which include any carbon-based molecule in the form of a hollow structure, were first produced experimentally in September 1985. Among the spherical fullerenes, C60 and C70 are the most common. Fullerenes intercalated with alkaline metals including potassium, cesium, and rubidium have been shown to possess superconductivity, as have halogen-doped fullerenes. Unfortunately, while these carbon-based superconductive materials show great promise, they have low critical transition temperatures. For example, rubidium-doped C60 has a critical transition temperature Tc of 28 K, and ternary intercalated fullerenes doped with cesium and rubidium have Tc of 33 K. Moreover, these materials still incorporate metals or halogens, and in certain applications may present materials issues.
What are needed in the art are additional non-toxic superconductive fullerenes as well as methods for enhancing characteristics of superconducting fullerenes. For instance, metal-free superconductive fullerenes would be of benefit. In addition, a method for increasing the critical transition temperature and/or decreasing the required stabilizing magnetic field required for the onset of superconductivity would be of great benefit. Moreover, devices that incorporate superconducting fullerenes with such enhanced features would be of great benefit.